This invention relates to printed circuit boards. More particularly, it relates to a printed circuit board architecture for low noise operation for analog/digital circuits.
Printed circuit boards are typically wired so that ground signals share a common plane on one of the layers of the board. This technique works well for digital or analog circuit boards which employ the use of high speed or wide bandwidth signals. It provides for relatively good transmission line parameter characterization which affords better frequency response and minimizes transmission line reflections.
In a mixed analog/digital system, the resulting ground currents of the two disciplines are significantly different. The analog subsystem generally suffers from noise problems induced by the rapidly transitioning signals of the digital subsystem. Its noise floor is generally poorer than what could be achieved using the same circuits completely isolated from the digital signals. One means to alleviate this problem is to segregate the signals and design the card so that the ground plane is divided into separate analog and digital ground planes. The technique substantially reduces the interaction of the ground currents, and a reduction of the noise crosstalk between the digital and analog sections. As shown in FIG. 1, the ground layer is separated into an analog and a digital ground plane.
This invention represents a further improvement in low noise operation of a printed circuit board, particularly in a mixed analog/digital system.